Neither am I a biologist or anything close actually I use to flunk in my biology, I use to hate it because I have problem remembering things and names would confuse me all the time and spellings were like really crazy.
But recently I had a thought, how can milk evolve, problem is if baby organism isn't ready to drink milk it would have problems and even if can the complex system of production of milk in mother organism is a problem.
If milk or something like this is produced but not consumed by baby it's a waste and any animal that would waste the resources would actually die out compared to it's competitors because it would have to eat more or would have less energy to feed itself.
At the same time consumption of milk can't evolve before evolution of milk or milk like substance.
My idea for that is milk actually evolved as excretion, waste product, it wouldn't be like milk but a separate outflow of lesser toxins, now it's totally possible for babies of those mom that had evolved a different rout to lesser toxic excretion to feed upon that, giving them better chance of survival and overall pass on the genes.
Once this is established, it's very easy for milk to improve in quality over time to provide the baby organism to have better and better chance of survival, compared to it's competitors.